Direct-current (DC) motors have numerous applications, especially for vehicles having a battery voltage source. Such a motor requires a pair of magnets, one associated with the rotor or moving element, and the other associated with the stator. The continuous force required to cause movement of the rotor arises from continually repositioning the magnetic fields of the motor. In the past, both the rotor and the stator magnets have been configured as electromagnets. However, such an arrangement has the distinct disadvantage that electrical power must be coupled to the rotating rotor, which in turn requires reliance on brushes. The brushes may be a source of inefficiency, and adversely affect the reliability of the motor. With the advent of improved permanent magnets, and especially improved solid-state power control circuits, practical brushless motors have become very common.
In a brushless DC motor, the rotor includes a permanent magnet, thereby obviating the need for brushes to couple electrical energy to the rotor. The rotor permanent magnet interacts with a changing or rotating magnetic field produced by fixed stator windings, which are in turn energized by alternating or pulsating electric power produced by electronic switching circuits. The fixed stator windings provide the conditions for effective heat sinking of the windings to the motor structure. The electronic switching circuits associated with the motor are equivalent to a DC-to-AC converter.
One type of prior-art brushless DC motor includes two stator windings arranged as illustrated in FIG. 1 to produce mutually orthogonal magnetic fields when energized. One end of each of the stator windings 86, 88 is grounded. In such a brushless DC motor, the non-grounded end of each stator winding is alternately coupled to positive and negative direct-voltage sources +V, -V, by a pair of alternately switched power transistors arranged in a "totem-pole" connection. In FIG. 1, bipolar PNP transistor 70 has its emitter coupled to +V, and its collector connected to the collector of bipolar NPN transistor 72, and to the ungrounded end of stator winding 86. The emitter of transistor 72 is connected to -V. Similarly, the emitters of PNP transistor 74 and NPN transistor 76 are connected to +V and -V, respectively, and their collectors are connected together and to the ungrounded end of stator winding 88. Damping diodes 78, 80, 82, and 84 are coupled from the collectors to the emitters of transistors 70, 72, 74, and 76, respectively. In the context of brushless DC motors, such an arrangement with two transistors driving one end of a winding alternately positive and negative is termed "full-wave." By contrast, in a "half-wave" arrangement, one end of a stator winding is coupled to one terminal of a direct voltage source, and the other end of the winding is coupled by a single switch to the other terminal of the DC source. Thus, the full-wave control arrangement requires two switching transistors for each stator winding, while the half-wave control arrangement uses only one switching transistor for each stator winding. Reduced transistor count is desirable both reduced cost and for enhanced reliability. The full-wave arrangement, on the other hand, since it divides the supply voltage across two power switching transistors, tends to be used when the full supply voltage would tend to exceed the breakdown voltages of the transistors.
The stator windings by their very nature are inductive, since they are configured to form a substantial magnetic field, which stores energy. Among the considerations which must be taken into account in establishing a control switch topology for a brushless DC motor is that of limiting the voltage surge tending to appear across the switching transistors when the current in an inductive stator winding is being switched to a nonconductive or OFF state. In a totem-pole full-wave drive arrangement such as that of FIG. 1, a diode is coupled across each of the switching transistors, poled to carry the current which occurs due to the inductive voltage rise or "kick" when the other of the switching transistors switches OFF, and to return the energy contained in the "kick" to the power supply. The return of the stored energy from the winding to the supply is very desirable in order to reduce power consumption and thereby increase energy efficiency of the motor. However, the half-wave switching arrangement is not amenable to such a solution, since there is but one switching transistor; the best that can be done is to couple a diode across the inductive stator winding to recirculate the inductive current through the stator winding itself, thereby dissipating the energy in the winding and the diode. For this reason, the full-wave arrangement tends to be more efficient than the half-wave arrangement. It would be desirable to combine the cost and reliability advantage of half-wave switching of the stator current with the efficiency advantage of recirculating the inductive energy at switch-off to the power source.